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Did fist fights drive evolution of human face?

Current theory about the shape of the human face just got a big punch in the mouth.

Two University of Utah researchers proposed on Monday that the face of the ancestors of modern humans evolved millions of years ago in a way that would limit injuries from punches during fist fights between males.

Their theory, published in the journal Biological Reviews, is presented as an alternative to a long-standing notion that changes in the shape of the face were driven more by diet - the need for a jaw that could chew hard-to-crush foods such as nuts.

"Studies of injuries resulting from fights show that when modern humans fight, the face is the primary target," biologist David Carrier said. "The bones of the face that suffer the highest rates of fracture from fights are the bones that show the greatest increase in robusticity during the evolution of early bipedal apes, the australopiths."

These are also the bones that show the greatest difference between women and men in early human ancestors and modern humans, Carrier added.

In both apes and humans, males are much more violent than females, and most male violence is directed at other males, Carrier said. The violence underpinning the need for a more robust facial structure may have involved fist fights over females, resources and other disputes.

Australopithecus was a lineage that preceded our genus, Homo, and it emerged more than 4 million years ago in Africa. Australopithecus was bipedal, smaller than modern people and possessed a combination of ape and human characteristics.

"Comparing great apes such as chimps and gorillas to australopiths, what changed in the face was a reduction in the length of the jaws, a great increase in the robustness and strength of the jaws, molar teeth and jaw muscles, a substantial increase in the size and strength of the cheek bones, and an increase in the part of the face that surrounds the eyes," Carrier said.

The proportions of the hand that allowed for the formation of a fist and the great increases in the robustness of the face occurred early in our lineage, 4 million to 5 million years ago, at about the same time as the bipedal posture appeared, Carrier added.

Carrier said anthropologists have thought the new facial traits in the first bipedal apes were the result of a diet that included very hard objects, and the biomechanics of eating such food can explain many of these features. But he said recent analyses of wear patterns in teeth suggest most of these creatures did not eat hard objects.

The study by Carrier and Michael Morgan, a University of Utah physician, builds on their previous research highlighting the role they contend violence played in driving human evolution.

"I think our science is sound and fills some longstanding gaps in the existing theories of why the musculoskeletal structures of our faces developed the way they did," Morgan added.